1. Field of the Invention
The present invention relates to liquid level sensors. More specifically, the present invention relates to thermistor probes and circuitry for single point liquid level sensing and automatic pump control.
2. Discussion of Background and Prior Art
Liquid level detection is an important part of many industrial processes. When the level of a liquid used in a process is too low or too high, a pump may need to be activated to adjust the liquid supply. Exceptionally high or low liquid levels may indicate a malfunction of the process, perhaps with serious consequences if the process is not stopped or the malfunction is not corrected quickly by pump activation or other action. Automatic pump control is especially useful in such circumstances.
Automatic sensing of liquid levels is not new. In particular, thermistors, or thermal resistors, have been used to detect liquid levels that are too high or too low and send corresponding electrical signals. It is a characteristic of thermistors that their electrical resistance varies inversely with their temperature, and that when electrical current is passed through them, heat is generated. A rising liquid level can cool a current-heated thermistor by inundating it so that the liquid conducts excess heat away, thus causing the thermistor's temperature to drop. The drop in thermistor temperature matches a rise in thermistor resistance, as reflected by a voltage increase across the thermistor, signaling the higher liquid level.
Two patents issued to Cliffgard, U.S. Pat. No. 3,631,440 and 4,656,464, for liquid level detectors use the property of thermistors to detect liquid levels by the temperature change that results when the thermistor goes from a "wet" condition to a "dry" condition. The '464 patented invention includes error checking by shorting out a part of the circuitry only as the power is turned on or a switch is closed. Furthermore, the '464 thermistor probe is not checked directly.
Self-checking can be an important feature in certain applications since some liquids and process environments are hostile to level detectors. Corrosive and radioactive environments demand rugged, robust level detection equipment and circuitry. Furthermore, although a level detector may be built to function reliably in such environments, critical process control requires constant verification that the detector is working so that, if it should fail, damage resulting from the consequences of a rising or falling liquid level is avoided. Also, a liquid level detector should be able to detect both aqueous and organic liquids typically used in industrial processes to be truly versatile, and should also be compact enough not to interfere with the processing system.